function [rho,u,v,p] = U2Variables(U)
% U2VARIABLES - 将守恒形式的守恒U变作物理变量 rou, u, v,p
%   
if length(U)~=4
    error('wrong length')
end

rho = U(1);
u = U(2)/U(1);
v = U(3)/U(1);
%p = 0.4*( U(4)-rho*(u^2+v^2)/2 );       
p = 0.4*( U(4)- ( (U(2))^2 + (U(3))^2 ) / (U(1)*2) );
end
